Excitation System Technologies for Wound-Field Synchronous ...

Received July 30, 2019, accepted August 3, 2019, date of publication August 6, 2019, date of current version August 21, 2019.

Digital Object Identifier 10.1109/ACCESS.2019.2933493

Excitation System Technologies for Wound-FieldSynchronous Machines: Survey of Solutions andEvolving TrendsJONAS KRISTIANSEN NØLAND 1, (Member, IEEE), STEFANO NUZZO 2,3, (Member, IEEE),ALBERTO TESSAROLO 4, (Senior Member, IEEE),AND ERICK FERNANDO ALVES 1, (Senior Member, IEEE)1Department of Electric Power Engineering, Norwegian University of Science and Technology (NTNU), 7034 Trondheim, Norway2PEMC Group, University of Nottingham, Nottingham NG7 2RD, U.K.3Department of Engineering Enzo Ferrari, University of Modena and Reggio Emilia, 41125 Modena, Italy4Department of Engineering and Architecture, University of Trieste, Trieste, Italy

Corresponding author: Jonas Kristiansen Nøland ([email protected])

This work was supported by the Open Access funding granted from the NTNU Publishing Fund.

ABSTRACT Wound-field synchronous machines (WFSMs) are included in the majority of large powergenerating units and special high-power motor drives, due to their high efficiency, flexible field excitationand intrinsic fluxweakening capability.Moreover, they are employed in a wide range of high-end solutions inthe low-to-medium power range. This contribution presents a comprehensive survey of classical and modernmethods and technologies for excitation systems (ESs) of WFSMs. The work covers the fundamental theory,typical de-excitation methods and all the modern excitation equipment topologies in detail. It also includesa description of the state-of-the-art and the latest trends in the ESs of wound-field synchronous motors andgenerators. The purpose of the paper is to provide a useful and up-to-date reference for practitioners andresearchers in the field.

INDEX TERMS Brushless exciters, de-excitation methods, excitation systems, exciterless excitation,harmonic excitation, rotating exciters, static exciters, synchronous machines, synchronous generators,synchronous motors.

I. INTRODUCTIONWound-field synchronous machines (WFSMs) are the pre-ferred choice in power generation applications ranging fromfew kVA to few GVA [1]. The main reasons are:

1) the possibility to control the flow of reactive power(both absorption and production);

2) the intrinsic flux regulation capability;3) the reliability and resilience to short-circuit faults with

no demagnetization risks;4) the high efficiency;5) and the superior dynamics during electro-mechanical

transients.

WFSMs are predominant in grid-connected operations[2] and in small-to-medium generating systems for isolatedapplications [3], [4]. Moreover, wound-field synchronousmotors are still today the preferred choice for high-power

The associate editor coordinating the review of this manuscript andapproving it for publication was Xiaodong Sun.

applications in the multi-MW range [5], [6], especially in theoil-and-gas industry [7]–[9] and for large shipboard propul-sion [10]. Although these machines represent a consolidatedand mature technology, much attention is currently given totheir design [11], analysis [12], modelling [13], as well asto their field excitation technologies [14]. While most of theresearch on permanent magnet (PM) synchronous motor andgenerator performance enhancement relies on sophisticatedcontrol algorithms [15], [16], a key role in WFSM develop-ment is played by new hardware and design solutions forthe improvement of rotor excitation in terms of dynamicperformance [17], reliability [18], compactness [19]–[22] andcondition monitoring [23]–[25].

The art of ESs for WFSMs is an old engineering dis-cipline. Fig. 1 shows a high-level sketch of its develop-ment history. Its first dramatic change happened in the1950s as a result of advances in high-power semiconductordevices. This triggered the replacement of electromechani-cal regulators, based on direct-current commutators [26], by

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J. K. Nøland et al.: Excitation System Technologies for WFSMs: Survey of Solutions and Evolving Trends

FIGURE 1. Development history of excitation systems for wound-fieldsynchronous machines.

silicon-controlled rectifiers (SCRs) and analogue controllers[27], [28]. Another paradigm shift happened in the 1980s,when controllers migrated from analogue to digital technol-ogy [29]–[32].

Over the last 20 years, the evolution of power electronicsand wireless technologies has enabled further innovationsand possibilities in the area. Currently, the static excitationmethods (in which the field winding is fed through brushesand shaft-mounted slip rings) are preferred whenever a fastdynamic response is required. However, they cause main-tenance and safety issues due to brush wear and possiblesparking. Brushless exciters (where power is supplied to thefield winding by electromagnetic induction with no slidingcontacts) overcome these challenges. On the other hand,they suffer from worse dynamic performance, prevent directaccess to the field winding for measurement and protec-tion purposes, and require the installation of an auxiliary

rotating machine (exciter) on the main motor or genera-tor shaft. Nevertheless, such limitations are being signifi-cantly reduced by the advances currently in progress. Forexample, integrated-harmonic brushless exciterless solutionsare a promising trend for the most compact options onthe market [19]–[22]. In addition, wireless communicationcan be nowadays used to measure field winding quantities[33], and employed to regulate the excitation current throughshaft-installed, remote-controlled power electronics devices[34]–[41], thus overcoming some limitations of brushlessexciters.

The main focus of this paper is to provide a comprehensiveand general review of the different excitation architecturesfor WFSMs. The fundamental theory of field excitation, stepresponses and de-excitation is first recalled in Section II.Section III presents an extensive survey of the possible exci-tation technologies presently available or under development,pointing out their pros and cons. Finally, Section VI con-cludes the paper by summarizing the work and discussing themost promising perspectives for future development.

II. THE FUNDAMENTALS OF FIELD EXCITATIONAn ES encompasses the equipment used to provide fieldcurrent to a synchronous machine, including power regula-tion and control, as well as protective elements [42]–[48].The ES fits the basic construction principle of a WFSMs,which include a set of rotating poles equipped with a fieldwinding fed by direct current and a stationary armature wind-ing which carry alternating currents. The operating princi-ple of the WFSM is extensively described in the technicalliterature [49].

The field winding of the WFSM is subjected to the ESoutput, which is the field voltage. The latter is referred to asUf 0 at no-load and Uf at rated conditions. The dynamics ofan ES is strongly influenced by the field winding inductance,which can be regarded as a constant except for magneticsaturation effects. The relationship between the field windinginductance (Lf ) and the field winding resistance (Rf ) in open-circuit conditions is

Lf = T ′doRf , (1)

where T ′do is the d-axis transient open-circuit time constantof the generator, or the time constant of the rotor in no-loadconditions. During steady-state operation, the field windingneeds to be supplied with a (constant) rated voltage Uf 0 anda (constant) rated current If 0 satisfying the following equation

Uf 0 = Rf If 0. (2)

The dynamics of the field winding during no-load operationis governed by a first-order differential equation, i.e.

uf = Rf if + Lfdifdt. (3)

where uf and if are the instantaneous field voltage and fieldcurrent, respectively. The automatic voltage regulator (AVR)

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FIGURE 2. Illustration of an ideal field current step response from 0.95puto 1.05pu, for several ceiling voltage values. The time is normalized withrespect to T ′

do for no-load conditions and to T ′

d for on-load conditions(first approximation).

typically acts in the per-unit frame, where the field voltage isexpressed as a fraction of the rated field voltage, i.e.:

uf = γfUf 0 (4)

where γf is the instantaneous per-unit field voltage. Thegeneral solution of (3) yields

if = γf If 0 + Ke−

tT ′do , (5)

where K is a constant depending on the initial field currentvalue. The damper cage effects are neglected, as they onlyaffect the very first instants of electromagnetic transients.During the on-load operation, the contribution of stator cur-rents needs to be taken into account. As result, the timeconstant during on-load operation takes a value which, to afirst approximation, can be assumed equal to the short-circuittime constant T ′d [50].

A. EXCITATION CURRENT DYNAMICSThe ceiling voltage is the maximum voltage that can beprovided by the ES under defined conditions [43]. The per-unit value of the ceiling voltage, with respect to the rated fieldvoltage, will be indicated as γceil below. An ideal ES must beable to apply the ceiling voltage instantaneously, in order toproduce the fastest possible change in its excitation current.Based on the solution of (5), Fig. 2 shows the impact ofthe available ceiling voltage γceil on the step response for atypical dynamic performance test [18], [51]–[54]. The idealresponse is shown, where the step change of the field voltageis instantaneous [43]. A frequency response test is an alter-native test, which will provide similar dynamic informationabout the ES [55].In practice, the voltage applied to the winding reaches its

maximum (ceiling) value in a finite time, which depends onthe AVR response delay and on the circuitry between the AVR

and the field winding according to the architectures that willbe described in the following sections. Any ES is classifiedas high initial response (HIR) when it can reach 95% of thedifference between ceiling voltage and rated field voltage in0.1s or less under specified conditions [43].

Naturally, the higher the ceiling voltage, the faster the cur-rent response will be. However, the ceiling voltage amplitudeis limited by various factors and, in particular, by the voltagewithstand capability of the field winding insulation [17].

In the example shown in Fig. 2, the field current is sup-posed to be initially at 95% of its no-load value, i.e., if (0) =0.95If 0, and it is supposed that the ceiling voltage is applied tothe field to increase the field current to a new set-point valueof 1.05 pu. In such case, the particular solution to (5) is:

ifIf 0= γceil + (0.95− γceil)e

−t

T ′do . (6)

The time 1 t taken by the field current to reach the set-pointvalue of 1.05pu in the open-circuit conditions, normalizedwith respect to the open-circuit time constant, is:

1tT ′do= ln

[0.95− γceil1.05− γceil

]. (7)

Fig. 2 shows the field current response for various possiblevalues of the per unit ceiling voltage γceil from 1.5 to 4. It canbe noticed from the figure that the field current dynamics isstrongly affected by the ceiling voltage if the latter is low,but much less for ceiling voltage values above 2 times therated field voltage. In other words, the speed of response isstrongly dependent on the ceiling voltage for lower values,but less dependent for values of γceil above 2 per unit.

Eqs. (6) and (7) can be modified for on-load conditionsby replacing T ′do and If 0 by T ′d and If . In fact, the sameconsiderations as for the no-load operation can be made. Theuse of T ′d rigorously applies to short-circuit conditions, but isalso useful to describe the on-load step response dynamics toa first-order acceptable approximation [50].

Fig. 2 presents normalized (per unit) quantities. As a result,it applies to both no-load and on-load conditions. For thelatter case, it is worth recalling that the base field voltagesand currents uf and if under nominal load are typically 1.5 to2 times their no-load values.

B. DE-EXCITATION METHODS AND DYNAMIC RESPONSESThe same analytical approach discussed in the previous sub-section can be employed for analyzing step responses of thefield current in the opposite direction, i.e. when the fieldcurrent has to be reduced or extinguished. However, in thiscase, the available negative ceiling voltage that can be appliedto force the field current to zero may be slightly less than thepositive one for certain types of ESs, as discussed in moredetail below.

The de-excitation response is an important property of anES. The energy stored in the field winding must be dissipatedas fast as possible in case of a normal or forced stop. In fact,since the field winding is a highly inductive circuit, a sudden

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FIGURE 3. Simplified de-excitation circuit of the two maindemagnetization methods.

interruption of the current flowing through it (i.e. by openinga breaker) would result in dangerous over-voltages that mayexceed the field insulation withstand capability. Therefore,de-excitation is mandatory to reduce the field current toapproximately zero before switching off the ES.As discussed in the next section, the capability of quickly

extinguishing the field current depends on the technologyand, in particular, is a peculiar strength of static ESs whencompared to brushless ones. A fast-response demagnetizationis obtained either by feeding the field winding with a negativefield voltage or short-circuiting it with a parallel-connecteddischarge resistor. Both methods are illustrated in Fig. 3.

1) DE-EXCITATION BY NEGATIVE CEILING VOLTAGEWhen the WFSM operates at on-load condition, a negativeceiling voltage results in a normalized de-excitation currentresponse described by (8), where γceil,neg is the applied nega-tive ceiling voltage. The time T37% taken by the field currentto reach 37% of the nominal value is as in (9).

ifIf= γceil,neg + (1− γceil,neg)e

−tT ′d (8)

1T37%T ′d

= ln

[1− γceil,neg1e − γceil,neg

](9)

Equation (9) is normalized with respect to T ′d since the on-load operation is typically the case of interest when evaluatingthe de-excitation performance. If the no-load de-excitationtime is considered, T ′do must be used and 1T37% takes alarger value. 1T37% is often imposed by some standards orregulations not to exceed a given threshold as a safety-criticalrequirement [56]–[58]. Fig. 4 shows how the de-excitationtime varies for negative ceiling voltages between 2 and 4 pu.

2) DE-EXCITATION BY DISCHARGE RESISTORA discharge resistor is a safety device designed to shortenthe time needed to extinguish the field current. A short-circuit of the field winding by the discharge resistor underload conditions leads to a de-excitation time constant givenby (10), where Rd is the discharge resistance. The field cur-rent dynamics will be then governed by (11) when startingfrom an initial field current equal to 1 per unit.

τd =T ′d

1+ RdRf

, (10)

FIGURE 4. Illustration of an ideal de-excitation field current responsefrom 1pu to 0pu for several negative ceiling voltage values. The time isnormalized with respect to T ′

do for no-load condition and to T ′

d foron-load condition.

FIGURE 5. Illustration of an ideal de-excitation field current responsefrom 1pu to 0pu, for several ohmic values of the discharge resistor,increasing from

RdRf

= 1pu toRdRf

= 10pu. The time is normalized with

respect to T ′

do for no-load condition and to T ′

d for on-load condition.

ifIf= e−

tτd = e

−tT ′d

(1+ RdRf

)(11)

Moreover,1T37% decreases as the discharge resistance growsaccording to (12):

1T37%T ′d

=1

1+ RdRf

. (12)

Fig. 5 represents how the de-excitation current responsechanges for discharge resistances varying from Rd = Rf toRd = 10Rf . The diagram shows that a discharge resistor hav-ing a high resistance value compared with the field windingwould be able to generate high initial de-excitation voltages.For this reason, in practice, the field winding insulation limitsthe maximum discharge resistance that can be used.

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TABLE 1. Summary and comparison of the static excitation systems.

The maximum de-excitation voltage also influences thechoice of the field circuit breaker or excitation interrup-tion devices as described in detail in [17], [56]. Consider-ing all these restrictions, practical values of Rd /Rf usuallylie between 0.5 and 2.0 [57]. As an alternative to conven-tional resistors, metal oxide or silicon carbide varistors area good solution to reach faster de-excitation times with lim-ited over-voltages. These are often referred to as non-lineardischarge resistors, due to their non-linear voltage vs. currentcharacteristic.

III. EXCITATION SYSTEM TECHNOLOGIESAND THEIR EVOLUTIONAs shown in Fig. 1, the first ESs were based on DC exciters[59]. These use DC generators as a power source, driven by aseparate motor or by the main alternator shaft, and supply DCcurrent to the field through slip rings. DC excitation methodsrepresent early commercial systems (from the 1920s to the1960s) and have, by now, a merely historical value, as theyhave been superseded by AC exciters. The most commonlyemployed ESs since the 1960s can be classified into threecategories: static, brushless and embedded (or integrated)systems. These three categories will be discussed in the nextsubsections in more detail.

A. STATIC EXCITATION SYSTEMSStatic exciters are dominantly used for large synchronousgenerators with power ratings in the order of several MVAs,which need to satisfy standardized technical requirementsin terms of dynamic performance [43]–[45]. Figure 6 illus-trates a classic example of this type of system. It includestwo six-pulse SCRs bridges (i.e. SCR1 and SCR2 in thefigure) connected in parallel to the WFSM field circuit,each of them with its own rectifier controller (C1 and C2).In this configuration, the SCR bridges usually operate in ’hotstandby’, i.e. one of them conducts the full excitation current,while the other remains in standby. When a failure occursin the active bridge, then it is automatically disabled andthe standby one takes over without any manual intervention.As an alternative approach, active current sharing betweenthe rectifiers of these systems has been proposed [60]–[64].

The other elements in the system are the field discharge (FD)system, the field circuit breaker (FCB) and the AVR, whichoutputs the commands to the SCR bridge controllers in orderto maintain the main machine voltage at its desired valuethrough a feedback control loop.

The two classical types of static exciters, namely the poten-tial source and the compound source, are illustrated in Fig. 7[65], while, Fig. 8 presents some recently developed or pro-posed static systems. In addition, Table 1 compares all thesestatic topologies and summarizes their main features.

Static exciters perform the de-excitation task by applyingnegative ceiling voltage obtained by controlling SCRs witha firing angle above 90◦ or through a discharge resistor. Thelatter is a passive component that can operate independentlyof the ES supply and it is often required, as it providesa reliable and independent de-excitation in case of severefailure of the AVR or faults within the SCR bridges [56].

1) POTENTIAL SOURCE (A1)The potential source or bus-fed ES is the most popular choicein the power industry [66]. Topology A1 in Fig. 7 illustratesits main components, including brushes and slip rings, whichrepresent the interface between the rotating and the stationarydomains, the potential transformer (PT) and the six-pulseSCR bridge. The PT steps down the three-phase voltage fromthe generator terminals and feeds the SCR bridge. The latterrectifies the three-phase voltage and supplies DC currentto the field winding. The rectifier output voltage amplitudeis controlled by the AVR to achieve its multiple regulationpurposes, such as control of the terminal voltage, limitationof field and terminal currents, reactive power exchange withthe grid, generator load angle restriction and power systemstabilization.

The ceiling voltage of the potential source exciter is verysensitive to the bus voltage. As a result, the PT secondary volt-age should be carefully chosen to satisfy the dynamic require-ments on the grid-side. The six-pulse SCR can attain 95%of the available ceiling voltage in less than 0.1 secondsand is thus considered a HIR ES [44]. A fast inherentresponse is essential for power system transient stability [67].As previously mentioned, a discharge resistor (Sec. II-A) is

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FIGURE 6. Components of a static excitation system for a 45 MVA, 514.3 r/min, 13.8 kV, 60 Hzsynchronous generator (Courtesy of Voith Hydro).

FIGURE 7. The two classical static excitation configurations.

usually connected to the SCR bridge output to ensure fast de-excitation (Sec. II-B).

2) COMPOUND SOURCE (A2)The compound-type static ES combines the generator termi-nal voltage and the contribution from the phase currents via

current transformers (CTs), as illustrated in topology A2 inFig. 7. During a fault, the PT will not be able to sustain theexcitation voltage. However, the fault current will provideexcitation power via the CTs to compensate for this. Thecompound system is an ingenious way to overcome some ofthe disadvantages of the potential source excitation. Its use is

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FIGURE 8. Modern extensions of the static excitation system, from community research and the industry.

particularly recommended in low-inertia systems. Exam-ples of those are isolated grids, e.g., shipboard power systems[68] or industrial plants, or large generators connected to rel-atively weak grids, where the voltage cannot be sustained byother sources in the presence of a fault [44]. High investmentcosts and complex circuitry are some of the main drawbacksof the compound technology, and an alternative solution isto design the potential source system with a higher ceilingvoltage [65], whenever this is technically feasible and eco-nomically convenient. In order to overcome the limitationsof systems A1 and A2, alternative solutions are proposed inthe recent literature, and new solutions have been tested bymanufacturers. These are described in the next subsections.

3) HYBRID PMSM (A3)The hybrid-type static exciter is the topology A3 in Fig. 8.In this arrangement, the PMs installed on the rotor of thegenerator provide the rated flux to themachine at no load. Thefield winding is, instead, used to compensate for the armaturereaction during on-load operation [69]. As a result, the hybridsystem could be classified as a PM synchronousmachinewitharmature reaction compensation. The self-excitation capa-bility has obvious advantages in islanded operation and inpresence of weak grids, thanks to the relatively low excitation

current that is required. However, the need for PMsmakes thegenerator design more complicated and expensive.

4) BOOSTER (A4)The static excitation booster, i.e., topology A4 in Fig. 8)is intended to improve the fault ride-through capability ofthe classical potential source exciter [70]–[72]. The DC-linkultra-capacitor is normally bypassed by a diode. During avoltage sag event, the insulated-gate bipolar transistor (IGBT)is activated and the ultra-capacitor is connected in serieswith the output of the SCR bridge. This is another exampleof a tailor-made solution to overcome some limitations ofthe potential source excitation aiming to improve the tran-sient stability of the power grid [51]. Note that the topologyA4 is different from the ’transient excitation boosting’ systemproposed in the early 1990s [74], which consisted of verypowerful static exciters.

5) BOOST-BUCK (A5)The boost-buck chopper configuration of the static ES isshown in Fig. 8 as topology A5 [75] in one of its possibleimplementations. It is another alternative to solve the fieldforcing capability of potential source systems in fault orlow grid voltage conditions. This configuration employs a

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FIGURE 9. Components of a brushless excitation system for a 400 kVA, 1500 r/min, 400 V, 50 Hz(175 Hz for exciter) synchronous generator.

boost-buck converter which can always provide the DC-linkceiling voltage at its output even when the terminal voltageis temporarily suppressed. This is possible thanks to theDC-link capacitor acting as an energy buffer that preventstransients in the power supply from immediately affecting theES output.

In addition, unlike an SCR bridge, the boost-buck con-figuration can turn off its switches at any given instant andsynchronization of its controller to the grid voltage is not nec-essary. This allows the ES to be fed by almost any availablepower supply, including a pilot machine, the AC auxiliarysystem, or even the DC auxiliary system. As an example,[75] proposes the boost-buck topology (shown in Fig. 8-A5)fed via an excitation transformer and a three-phase diodebridge. The solution is expected to be expensive for largesynchronous generators, which demand a high direct currentto be fed by a diode rectifier together with other powerelectronics apparatus. The compact thyristor based topology(Fig. 6) is the most mature and well-established technologyfor those applications [76], [77].

6) VOLTAGE SOURCED (A6)In Fig. 8, topology A6 illustrates a static exciter with afront-end VSC and an output dual-quadrant chopper [73].

The presence of a DC-link voltage and capacitance (withshort-term energy storage capability) provides a stronger fieldforcing capability which strengthens the transient stabilityof the WFSM. In addition, the VSC can operate in boostmode and the output chopper provides a faster field voltageresponse compared to the SCR bridge.The de-excitation process starts immediately by interrupt-

ing the PWM signals at the H-bridge chopper output. As aresult, it eliminates a possible commutation failure by theuse of IGBTs. Typically, there are challenges in design-ing a totally passive, fail-safe trigger circuit for the dis-charge resistor as recommended in [44]. Considering thatthe field winding energy can reach several MJs in largegenerators, the DC-link would require a large capacitor. How-ever, the VSC typically controls the DC-link voltage andwill actively discharge the capacitor during the de-excitationprocess.

B. BRUSHLESS EXCITATION SYSTEMSThe basic and most common structure of a brushless ES isillustrated in Fig. 9. It consists of a rotating exciter witha three-phase winding on the rotor, which is mounted onthe main machine shaft. The exciter field winding is wound

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TABLE 2. Summary and comparison of the brushless excitation systems.

around stator poles and is supplied by a DC source, althoughsome AC-fed stator designs may be required in synchronousmotors to provide field current at stand-still as discussed lateron. When the shaft revolves, the main air-gap field induceselectromotive forces and thus currents in the exciter rotorphases. A rotating AC-to-DC rectifier, assembled onto theexciter rotor, rectifies the induced AC current into DC currentand feeds the rotor field winding of the main WFSM.

Traditionally, the use of a rotating exciter and a dioderectifier is considered unfit to deliver prompt field voltagecontrol, as required for HIR ESs [78]–[80]. Moreover, untilrecently, the de-excitation methods described in section II-B have been regarded as unsuitable for implementation inbrushless ESs. In contrast, the speed of response is not astringent requirement in applications such as standby power,marine, oil and gas, UPS mobile constructions, etc. [81].In those, generators ranging from few kVA to few MVA areoften designed to feed mainly passive ohmic-inductive loads,for which AVR fast response and de-excitation capabilitiesare not major concerns. Not least, the use of slip rings andbrushes is not allowed in potentially explosive environments,which limit their application. Furthermore, they demand con-stant maintenance due to mechanical wear and tear.

Owing to the above, brushless exciters represent a goodsolution for applications requiring flexible excitation control,limited maintenance and operational costs. As such, theynowadays represent the most common configuration appliedby industry in small-to-medium generator sets [82]. Fig. 10

presents the classical brushless ESs as well as new topologiesintroduced in recent times, or still under development andresearch. Finally, Table 2 compares all the brushless topolo-gies taken into account by this work.

1) SHUNT (B1)The shunt-connected brushless exciter is a self-magnetizedsystem [83], shunted on the WFSM terminals. It uses the sin-gle line-to-line voltage of the generator terminals to power theAVR [84]. The same output is measured and used as a feed-back signal to regulate the generator voltage. The approachis cheap and simple to implement. However, the AVR sup-ply power is very much dependent on load-side events suchas voltage disturbances and faults. Also, when non-linearloads (e.g. power electronics based loads such as PCs, flu-orescent lamps and inverter-fed motors etc.) are fed by themain generator, the AVR supply power quality is compro-mised [82]. The solution cannot tolerate high overloads,neither can it offer sustained short-circuit operation capa-bility. In the worst case of a short-circuit fault occurringin the proximity of the machine terminals, the voltage maydrop to zero and powering the AVR would become chal-lenging if no precautions are taken. Apart from the men-tioned issues, the shunt solution is the simplest and cheapestamong the brushless systems described in this section. Theexcitation method is typically implemented in WFSMs ratedup to 700 kVA, but it is not uncommon for several MVAsas well.

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2) EBS (B2)The excitation boost system (EBS) is a boosted version ofthe shunt-type ES. A small excitation boost generator isinstalled on the WFSM shaft to power the excitation boostcontroller (EBC), which is an extension of the classical AVR[85]. The EBS has its terminals connected in parallel to theoutput of the main AVR and is activated by an AVR com-mand. The EBS only intervenes when needed, e.g., in caseof faults, load injections, etc. allowing the required excita-tion power to be developed even under these circumstances.It is not widespread in applications that require continuouspower generation, but more common in emergency, back-up or safety-critical applications. Moreover, it is compatiblewith non-linear loads such as power electronics based devicesand motors at startup and can provide excitation for 300%sustained short circuit current or more.

3) PMG-SCR (B3)The PMG-type system is a separately-excited arrangement,as opposed to the classical self-excited shunt-type topology[86]. It employs an auxiliary electrical machine, the PMGinstalled at the end of the WFSM shaft, to provide powerto the AVR when the shaft is running [87], [88]. The AVRpower is completely independent of the WFSM terminalvoltage and of the type of load it supplies. Another majoradvantage compared to the self-excited shunt topology is thatthe PMG provides more reliable power to the AVR and safervoltage build-up, which does not rely on residual magnetismbut is assured by the PMs. Furthermore, it enjoys a highoverload capacity and is suitable for demanding applications.In the SCR-type of the PMG system, the PMG is equippedwith a three-phase stator winding connected to a 4-pulsediode bridge rectifier which, in turn, feeds a single SCRdevice. Only two phases of the PMG are utilized in thiscase, thus avoiding the design of a dedicated single-phasePMG. By acting on the SCR firing angle, the AVR adjusts thesupply voltage applied to the rotating exciter stator terminals.A freewheeling diode connected in anti-parallel with the SCRensures that the exciter supply current can flow while theSCR is in blocking mode. The switching frequency of theSCR is two times the fundamental electrical frequency ofthe PMG stator. As a result, rapid dynamics for the excitersupply voltage is difficult to obtain [89]. However, a simpledroop control of the AVR can be easily implemented [90].The PMG-type is the most common ES for non-linear loads,in applications where the grid voltage is affected by signifi-cant disturbances typically due to power electronics devicesand motor starting. The PM machine obviously adds weightand overall complexity to the system. As a result, it is mostlyused for medium-size generator sets rated between 700 kVAand 4 MVA.

4) PMG-PWM (B4)The PWM-type of the PMG ES employs a three-phase PMGthat supplies a DC-link capacitor via a 6-pulse diode rectifier

[89], [91], [92]. An IGBT regulates the exciter stator currentusing PWM technique [91]. In general, it is a variant ofthe PMG-SCR topology, thus offering similar benefits. How-ever, it provides a higher power quality to the exciter statorwinding since PWM harmonics have higher frequencies andare thereby attenuated by the exciter winding inductance,resulting in less current ripple than in the PMG-SCR case.

5) AUXILIARY SOURCE (B5)The auxiliary source brushless ES [25], [93]–[95] is a clas-sical solution for large grid-connected WFSMs, where thestator winding of the main exciter is fed from an auxiliarylow-voltage grid available for the ES. The main exciter couldalso be fed by the main grid via an excitation transformer anda thyristor rectifier [96]. The solution is simple and robust,but implies the availability of a suitable auxiliary powersource or of a strong main grid. In safety-critical nuclearpower applications, an exclusive exciter with 39 phases canbe used to improve the reliability and fault tolerance of theES [24], [96].

6) AUXILIARY WINDING (B6)The auxiliary winding solution is characterized by the use ofan auxiliary, single-phase winding located inside the WFSMstator slots [97]. The single-phase winding is connected toan isolating transformer that feeds the exciter stator wind-ing through a single SCR device controlled by the AVR.When the WFSM operates at no load before synchroniza-tion to the grid, an initial excitation must be provided. Forthis purpose, PMs embedded in the exciter stator are used.The auxiliary winding based ES is less common than thepreviously described ones and is typical of power ratingsbetween 4MVA and 15MVA. This excitation method pro-vides power to the AVR independently of the load type,as in separately-excited systems (i.e., equipped with PMG).However, the voltage induced in the auxiliary winding isproduced by the main air-gap flux and therefore depends onthe WFSM operating condition. For this reason, the design ofthis kind of ES is a challenging task. The auxiliary windingeliminates the need for the PMG, thus resulting in a verycost-effective solution. On the other hand, the additionalauxiliary winding reduces the useful space for installing themain stator three-phase winding, potentially resulting in anincreased size of the WFSM. Note that there are possibilitiesfor usingmore auxiliary windings. In the field of synchronousmotors, they can be employed to obtain a bearing-lessconfiguration [98], [99].

7) AE (B7)Wound-rotor synchronous motor for variable-speed drivestends to employ AEs [100]. Moreover, civil aircrafts useelectrically excited generators for fixed speed and variablespeed applications [101], [102]. The exciter behaves as athree-phase rotating transformer. As a result, it can deliverfield current to the main WFSM even at standstill conditions.In high-power variable-speed drive applications, the AE is

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traditionally fed by an AC/AC anti-parallel thyristor con-verter, due to economic constraints, while the main statoris usually fed from either a current-source or voltage-sourceinverter. Due to the high complexity of the system, efficientmodeling has been proposed [103]. Moreover, measurementof the field current is not straightforward without brushesand slips rings or without fast wireless data transmission.As a result, field current estimation and control methodshave been a research focus [104]–[106]. Good accuracyin the field current estimation ensures superior dynamicand steady-state performance. In addition, estimation of therotor position by high-frequency voltage injection and bymodel based approaches have recently been addressed [107],[108]. Two-phase AEs have also been proposed in integratedstarter/generator applications [109], where the motor opera-tion capability removes the need for a separate starter, suchas in aircraft onboard generators. The AE can provide fieldcurrent for full load starting of the main WFSM when therotor is at standstill.

Figure 8 shows the AE with an auxiliary source. However,the AE system could also work with a pre-exciter PMGconfigured in a three-stage system [110].

8) RT (B8)The rotating transformer (RTs) electrically-excited config-uration is a competitive choice in vehicle applications fordeep flux weakening, high torque and efficiency at lowspeeds [111], [112]. The RT based brushless exciter isessentially a single-phase AE in the axial rotary config-uration. However, it can also be designed in a pot coreconfiguration [113]–[115]. It has a passive 4-pulse diodebridge rectifier in the rotor and an active-switch powerelectronics supply for the exciter stator winding. The useof high frequencies for the exciter stator supply has beenshown to be beneficial for compactness in vehicle applica-tions [116], enabling wireless power transferring technol-ogy [117]. The cost and supply volatility of rare-earth PMshave intensified the interest for RT based WFSMs in vehicleapplications [112].

9) SHUNT HYBRID (B9)To overcome the limitations of the shunt excitation methodwithout resorting to a separate PMG solution [87], a hybridexcitation for the exciter is presently under investigation anddevelopment. The PMs are installed in the exciter stator inorder to provide the ‘‘basic excitation power’’ to the mainmachine during its no-load operation. In this way, there isno need to modify the circuitry feeding the exciter and/orthe generator field windings. This solution can lead to asignificant field loss reduction in the exciter in any loadingcondition of the WFSM, thus resulting in improved thermalmanagement of the system. In addition, the PMs ensurecontinuous operation of the system even in case of a faultoccurring in the exciter field winding, thus improving theoverall reliability. Also, the voltage build-up process duringstarting is improved with this solution, as it does not rely upon

the ferromagnetic core residual magnetism as in the shuntexcitation method. However, the WFSM terminal voltagecannot be fully controlled from zero at rated speed, whichis needed for ESs employed in test-field applications. TheSCR based AVR compensates for the armature reaction underloaded conditions.

10) HIGH-SPEED DE-EXCITATION (B10)One of the main drawbacks of the conventional brushlessESs is the difficulty of implementing the de-excitation strate-gies addressed in Section II. Recently, multiple tests havebeen carried out to provide a self-actuated high-speed de-excitation system for the conventional brushless arrangementin large power plants [50], [118]–[120]. A non-linear dis-charge resistor is usually bypassed in the rotor circuitry butis activated during internal short-circuits to avoid damages tothe machine.

11) TWO-STAGE PME-SCR (B11)In this configuration, the stator winding of the main exciter isreplaced by a PMs, yielding an outer pole PM exciter (PME)[37], [121], [129]. Two-stage means that there are only twomachines involved, i.e., the exciter and the generator. Allcontrol actions are accomplished by the SCR mounted onthe rotor. This technology has similar characteristics as theHSR dual-control, except for the fact that the ceiling voltagecannot be controlled due to the presence of PMs in the exciterstator. As a result, the available ceiling voltage of the ESwill be directly dependent on the loading condition of theWFSM. This is because the WFSM loading is reflected inits field current, causing a high commutation voltage dropacross the rotating SCR rectifier. Moreover, a high ceilingvoltage demands a high SCR rectifier firing angle, which hasbeen shown to generate torque pulsations resulting in possibledamages to the exciter stator [129].

12) HSR DUAL-CONTROL (B12)The high-speed response (HSR) dual-controlled ES is a com-mercial product for large SMs [34], [35]. A rotating 6-pulseSCR bridge rectifier is remote-controlled through a wirelesssystem and mounted on the rotor as an interface betweenthe exciter armature and the generator field winding. Thesolution performs two control tasks: 1) exciter field currentregulator (FCR) on the stator side; 2) AVR on the rotor side.It features the same dynamic performance of the conventionalstatic ES, but without the need for brushes and slip rings.In contrast, it is not directly sensitive to the generator terminalvoltage as the static systems. The ceiling voltage amplitudecan be controlled by the FCR, only depending on a low-powerauxiliary source, typically fed from a low voltage grid.

13) PME-SCR HYBRID-MODE (B13)The PME-SCR hybrid mode configuration is a multiphaseextension of the PME-SCR two-stage system [38], [122].It utilizes a dual-star PM exciter with a 12-pulse SCR rectifier,composed of two rectifier units connected in either parallel

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FIGURE 10. Overview of the classical brushless excitation configurations.

or series. The hybrid-mode topology typically works with aparallel connection in stationary conditions for low excitercurrents, causing small torque pulsations. However, the seriesconnection is activated with a single IGBT device to apply thefull ceiling voltage to the exciter. The system also providesboth active current sharing [123] and redundant post-faultoperation for enhanced power production reliability [122].

14) PME-PASSIVE WITH ROTARY CHOPPER (B14)There has been recent research trying to extend the classicalbrushless systems replacing the rotating diode bridge rectifierwith a rotating thin film capacitor and a modern output chop-per [36], [124], [130]. The system has the advantage of rela-tively few rotating active components. However, the DC-linkcapacitor must be designed to absorb the magnetic energy

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FIGURE 11. Modern extensions of the brushless excitation configurations, from community research and the industry.

stored in the field winding during a rapid de-excitationbecause the diode bridge is power-unidirectional.

15) CPT (B15)Although RTs (B8) provide a viable contact-less solution,the use of a Capacitive Power Transfer (CPT) removes the

need for additional windings and magnetic cores [125]. TheCPT is based on a rotating capacitor with relative motionbetween electrodes having constant active surfaces and airgap between them. Due to the small coupling capacitance inair, the solution is suitable for low-power excitation appli-cations [126]. However, hydrodynamic CPTs can provide a

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FIGURE 12. Embedded, integrated or exciterless excitation, both classical and modern.

larger excitation power [127]. Moreover, they represent anadvantageous technology in terms of weight, volume andconstruction simplicity. [131].

16) PME-VSC WITH ROTARY CHOPPER (B16)Finally, the PME VSC solution with a rotary chopper pro-vides a full-scale modern power electronics solution. Thechopper can demagnetize the field winding and the rotatingVSC can de-energize the DC-link capacitor. As a result,the size of the rotating capacitor can be significantly reduced.As a drawback, the number of active rotary componentsincreases [128]. Note that the rotating VSC control requiresdirect measurement or estimation of the rotor position.

C. EMBEDDED EXCITATION SYSTEMSThe embedded ES is an exciterless alternative where theexciter components are integrated into the main WFSM sta-tor. Although the idea is old [136]–[145], new solutions and

implementations have been recently proposed. Table 3 com-pares all the embedded topologies presented herein.

1) STATOR HARMONIC (C1)The stator harmonic winding system is an old classicalembedded system [132]. The excitation power is generatedby an additional single-phase harmonic winding embeddedin the stator of the main machine and designed so thatair-gap space rotating field harmonics induce an electro-motive force in it. This suffices to generate enough powerto feed the rotor field winding via a 4-pulse SCR throughbrushes and slip rings [20]. From a conceptual point of view,it is similar to the solution B6 shown in Fig. 10, with itspositive and negative sides. On the other hand, being anexciterless solution, the additional stator harmonic wind-ing must be able to provide the full excitation power tothe WFSM. This indicates that more space is necessary

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TABLE 3. Summary and comparison of the embedded or exciterless excitation systems.

for its allocation, thus resulting in a significant increase inthe active WFSM size.

2) ROTOR HARMONIC WITH RPE (C2)The exciterless system with rotating power electronics (RPE)embeds the whole ES circuitry in the rotor [133]. It utilizesrotor damper slot harmonics to induce the excitation powerin a single-phase auxiliary circuit embedded in the rotor. Theauxiliary winding injects current into a rotating DC-link viaa boost converter stage, and the DC-link voltage feeds theWFSM field winding. This solution does not require brushesand slip rings.

3) DOUBLE HARMONIC (C3)In the double harmonic system, a harmonic air-gap flux isgenerated by a stator auxiliary winding featuring a numberof poles different from the main WFSM. This energizes anadditional harmonic winding embedded in the rotor whichfeeds the field winding [20]. Although the technology hasinteresting features such as compactness, it depends on anauxiliary source to feed the stator harmonic winding viaa VSC.

4) ZERO SEQUENCE (C4)The zero-sequence system avoids brushes and slip rings via aconverter-fed stator that generates a zero-sequence harmoniccomponent that excites a harmonic winding embedded in therotor, which is used to feed the field winding [19], [134]. Thecompactness of this system is a significant strength. However,it requires advanced power electronics, which adds costs [20].A similar strategy utilizing sub-harmonic components hasbeen recently proposed [146], [147].

5) INDUCTION EXCITED (C5)The induction excited topology combines both an AE andthe main WFSM into the same system. The main WFSMand the AE are designed with a different number of polesto avoid a magnetic coupling between them [21], [22]. Sincethe exciter is an induction machine, the WFSM field windingcan be excited during standstill conditions, which makes itsuitable for generator/starter applications and for high-end

motor applications. The solution requires an auxiliary sourcethat feeds the induction machine via a VSC.

6) SYNCHRONOUS EXCITED (C6)In the synchronous excited topology, the main WFSM anda DC-fed brushless exciter are assembled together. They areconfigured for a different number of poles so that they donot interfere electromagnetically with each other [135]. Theadvantage is that this configuration can be excited by a single-switchDC-DC converter having a relatively low power rating.

IV. CONCLUSIONThis paper has covered all the main ES categories presentlyimplemented or proposed for WFSMs. A wide range oftopologies for each category has been described, highlight-ing state-of-the-art technologies as well as recent trends.The design of an ES involves different performance chal-lenges which are strongly influenced by the applicationat hand. These challenges include speed of response, de-excitation capabilities, field forcing requirements and startingperformance. Additional open issues relate to compactness,simplicity, safety, operational costs and maintenance. Thekey features of the different technologies are summarizedin Tables 1, 2 and 3.

In addition to traditional systems, new solutions are nec-essary to address the increasingly demanding performancerequirements often imposed on modern WFSM, such as:• post-fault operation for reduced downtime [122] andcompensation of cooling discrepancies [123];

• extended capability diagram and over-excitation supportunder long-term voltage instability [148], [149];

• excitation boosting for enhanced power system stability[73] and voltage-sag ride through capability [70];

• cancellation of unbalanced magnetic pull [150], [151];• mitigation of air-gap flux density harmonics [152],[153];

• faster high-torque starting by polarity inversion [154].The overview presented in this paper on traditional and

modern approaches to field excitation for WFSMs has shownthat there are many options available and the selection ofthe best system architecture strongly depends on the specificapplication requirements. Classical static exciters are still

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the preferred choice for most of large MVA generators, butnew brushless exciters are being introduced and developedas competitive alternatives. Conventional brushless excitersare dominant for smaller WFSMs, but harmonic integratedexciters are paving the way to more compact solutions.

In addition, thanks to the recent advances in the field ofrotating power electronics, and wireless rotor signal trans-mission and monitoring, ESs for WFSMs are expected tofurther evolve and meet new requirements and performancestandards. In such a wide and fast-evolving scenario, thiswork represent a useful reference for designers and prac-titioners to identify the most suitable solution for a givenapplication, especially in view of possible revisions of thecurrent standards on voltage regulation and control.

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JONAS KRISTIANSEN NØLAND (S’14–M’17)received the M.Sc. degree in electric power engi-neering from the Chalmers University of Technol-ogy, Gothenburg, Sweden, in 2013, and the Ph.D.degree in engineering physics from Uppsala Uni-versity, Uppsala, Sweden, in 2017.

In 2017, he became an Associate Professor withthe University of South-Eastern Norway (USN).Since December 2018, he has been an AssociateProfessor with the Department of Electric Power

Engineering, Norwegian University of Science and Technology (NTNU).He has been actively involved in the grid standardization of new excitationsystem technologies for the Scandinavian power system. His current researchinterests include smart excitation systems and their interplay with the powersystem, the optimal utilization of electrical machines, and starter-generatorsfor aircraft applications.

Dr. Nøland is a Board Member of the Norwegian Academic Committeeof Publication in Technology in Electrical Power Engineering. He is on theSteering Committee of the IEEE Power and Energy Chapter of Norway. He isalso a member of the IEEE Industrial Electronics Society (IES), the IESElectric Machines Technical Committee, the IEEE Industry ApplicationsSociety (IAS), and the IEEE Power and Energy Society (PES). He regularlyserves the scientific community as a Reviewer for several journals andconferences.

STEFANO NUZZO (M’18) received the B.Sc. andM.Sc. degrees in electrical engineering from theUniversity of Pisa, Pisa, Italy, in 2011 and 2014,respectively, and the Ph.D. degree in electricalengineering from the University of Nottingham,Nottingham, U.K., in 2018, where he is cur-rently a Research Fellow of the Power Electronics,Machines and Control (PEMC) Group.

Since January 2019, he has been a ResearchFellow of the Department of Engineering Enzo

Ferrari, University of Modena and Reggio Emilia, Modena, Italy. Hisresearch interests include the analysis, modeling, and optimizations ofelectrical machines, with a focus on salient-pole synchronous generators andbrushless excitation systems for industrial power generation applications.He is also involved in a number of diverse projects related to more electricaircraft initiative and associated fields.

Dr. Nuzzo is a member of the IEEE Industrial Electronics Society (IES)and the IEEE Industry Applications Society (IAS). He regularly serves thescientific community as a Reviewer for several journals and conferences.

VOLUME 7, 2019 109717


ALBERTO TESSAROLO (SM’06) received theLaurea degree in electrical engineering from theUniversity of Padua and the Ph.D. degree in elec-trical engineering from the University of Trieste,Italy, in 2000 and 2011, respectively.

Before joining the University, he was involvedin the design and development of large innova-tive motors, generators, and drives. Since 2006,he has been with the Engineering and ArchitectureDepartment, University of Trieste, Italy, where he

teaches the courses in electric machine fundamentals and electric machinedesign. He leads several funded research projects in cooperation with indus-trial companies for the study and development of innovative electric motors,generators, and drives. He has authored over 150 international papers in theareas of electrical machines and drives. He has been an Associate Editorof the IEEE TRANSACTIONS ON ENERGY CONVERSION, the IEEE TRANSACTIONS

ON INDUSTRY APPLICATIONS, and IET Electric Power Applications. He cur-rently serves as the Editor-in-Chief for the IEEE TRANSACTIONS ON ENERGY

CONVERSION. He is a member of the Rotating Machinery Technical Com-mittee TC2 (rotating electric machinery) of the International Electrotech-nical Commission (IEC). He is also a member of the IEEE Power andEnergy Society Electric Machinery Committee, the IEEE Industry Appli-cations Society Electric Machines Committee, and the IEEE IndustrialElectronics Society Electric Machines Technical Committee.

ERICK FERNANDO ALVES (S’06–M’08–SM’19) received the Engineering degree in energyand automation from the University of Sao Paulo,Brazil, in 2007, and the M.Sc. degree in electricalengineering from the Arctic University of Norway,Narvik, in 2018. He is currently pursuing the Ph.D.degree with the Department of Electric PowerEngineering, Norwegian University of Scienceand Technology, Trondheim, Norway, with a focuson virtual synchronous machine technologies. He

is currently pursuing the Ph.D. degree with the Department of Electric PowerEngineering, Norwegian University of Science and Technology, Trondheim,Norway, where he focuses on virtual synchronous machine technologies.

From 2007 to 2018, he held several positions at the Voith Group in Brazil,Norway, and Germany. During this period, he was involved in the design,engineering, commissioning, and development of excitation systems for over40 power plants in 18 countries. His latest assignment was with Voith DigitalVentures, Heidenheim, Germany, as the Product Manager, where he wasresponsible for power generation controls, in particular excitation systemsand turbine governors.

Mr. Alves is the Treasurer of the IEEE Power and Energy Chapter ofNorway. He is also a member of the IEEE Industrial Electronics Society(IES), the IEEE Industry Applications Society (IAS), the IEEE Power andEnergy Society (PES), and the IEEE Control Systems Society (CSS).

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